fix(systemd): handle starting containers that already exist

After a reboot, named containers for Deis components often
exist in a stopped state.  Prior to this commit,
Deis would fail to start the components due to a name conflict.
Now any existing named containers are removed in ExecStartPre
so ExecStart should always succeed.
Merge pull request #871 from deis/remove-on-start

fix(systemd): handle starting containers that already exist